In the city of Seattle, pro-cannabis campaigners celebrated in a haze at the foot of the Space Needle tower at one minute past midnight, the moment the state's new law came into effect.

Vivian McPeak, director of Seattle's annual Hempfest, said: "All our lives we've been living under the iron curtain of prohibition. The whole world sees that prohibition just took a body blow."

The new law allows cannabis to be smoked only inside: doing so in public is still subject to a $100 fine. However, the Seattle Police Department has told its 1,300 officers that, until further notice, they should not issue tickets.

On the city's police website, spokesman Jonah Spangenthal-Lee said: "The department's going to give you a generous grace period to help you adjust to this brave, new, and maybe kinda stoned world. The police department believes that, under state law, you may responsibly get baked, order some pizzas and enjoy a 'Lord of the Rings' marathon in the privacy of your own home, if you want to."

Washington and Colorado became the first two states to decriminalise and regulate the possession of cannabis, in ballots held alongside the US presidential election on November 6. The Colorado law takes effect on January 5.

In Washington, it is now legal for adults over the age of 21 to possess an ounce of the drug, or up to 16 ounces of cannabis-infused goods such as brownies, or up to 72 ounces in liquid form.

Growers and processors of cannabis will be regulated and the drug will be sold in licensed shops. It will be subject to a 25pc tax at each stage of that process.

The move is expected to bring in hundreds of millions of dollars in tax revenue, for spending in areas including schools and health care.

However, with cannabis still illegal under US federal law through the Controlled Substances Act, Washington state could face a crackdown by agents from the FBI and Drug Enforcement Agency.

The US Justice Department has yet to announce whether it will sue in the courts to try to block regulation of the cannabis industry in Washington and Colorado, which would set up a legal showdown over states' rights.

At the Space Needle, the mostly middle-aged cannabis smokers listened to reggae music from loudspeakers. A smoker, calling himself Professor Gizmo (50) said: "Victory for hemp. If our forefathers could see us now."